1. Field of the Invention
The present invention relates to a toner, a method for producing the toner, and an image forming apparatus.
2. Description of the Related Art
In recent years, low-end laser beam printers have increasingly been reduced in cost, downsized and elevated in response speed, with targeting end users. For such downsizing and cost reduction, downsizing each part and simplifying apparatuses have been required.
For example, in a developing device mounted to an image forming apparatus, a one-component developing process without using a carrier can make the developing device smaller than a two-component developing process using a carrier as a developer component. This is because the developing device employing the one-component developing process requires a smaller number of parts.
The one-component developing process is a process including: frictionally charging toner particles by a toner-regulating member; and forming a toner image on a toner bearing member using the charged toner particles. Although the one-component developing process requires a simpler structure than the two-component developing process, the toner itself is required to have various functions.
In particular, it is important for the toner to have high mechanical strength and high charging performance. In addition, the toner has to be resistant to external stimuli.
Employable methods for producing the electrophotographic toner having high charging performance include: (1) a method in which a charge-controlling agent is attached uniformly to the surface of each of the granulated toner particles; and (2) a method in which a charge-controlling agent is used in addition to toner materials such as a resin and a colorant in the production step of toner base particles.
The above method (1) is, for example, a method in which external additives such as silica, alumina and titania are externally attached to toner base particles (e.g., a method in which toner base particles and external additives are mixed with a mixer such as HENSCHEL MIXER). The externally-added charge-controlling agent, however, tends to be exfoliated from the resultant toner base particles. As a result, it is impossible to keep the charge amount of the toner for a long time, which is one existing problem.
The above method (2) is, for example, the chemical toner method or the knead-milling method using a charge-controlling agent and/or a resin having charge-controlling effects.
For example, Japanese Patent Application Laid-Open (JP-A) No. 11-174738 proposes a toner including: a binder resin; a charge-controlling resin having an ionic functional group; and a releasing agent having an ionic functional group with the opposite polarity to that of the charge-controlling resin, wherein the binder resin and the charge-controlling resin each formed of molecular chains with which the binder resin and the charge-controlling resin have affinity with each other.
However, since this proposed toner is produced by the knead-milling method, the charge-controlling agent is dispersed entirely in each toner particle. As a result, the amount of the charge-controlling agent is small in the vicinity of the surface of the toner particle, and the charge amount of the toner is not satisfactory.
When the amount of the charge-controlling resin or the charge-controlling agent used is increased to increase the amount thereof in the vicinity of the surface of the toner particle, the formed toner particles become hard to be degraded in fixability on recording media.
In the solution suspension method belonging to the chemical toner method, incorporation of the charge-controlling agent would make it difficult to form toner particles. Even if incorporated, the charge-controlling agent is incorporated closer to the center of each toner particle, so that it cannot exert its effect satisfactorily.
As has already been known, the aggregation method belonging to the chemical toner method produces a core/shell toner having a core made of resin superior in thermal fixation and a shell of resin superior in charging performance.
For example, JP-A No. 2008-089918 proposes improving charging performance of the surface of a toner by uniformly dispersing the charge-controlling agent in the shell layer of the toner, in order for the charge-controlling agent to exert its functions satisfactorily.
In this proposal, however, since the surface layer of the toner is entirely covered with the shell layer containing the charge-controlling agent, the thermal properties of the toner as a whole are changed, and the functions of the ingredients contained in the core are prevented. For example, during fixing, the releasing agent (wax) is prevented from bleeding towards the toner surface, degrading fixing performance of the toner. Also, when the core/shell toner is used as a one-component developer, the shell may be removed due to rubbing between toner particles and/or stress applied at a part for regulating the amount of the toner. As a result, there are problems such as degradation of the charging performance of the toner, fusion of the toner to a regulating blade, and filming on a developing roller.